The present invention relates to a power transistor module intended for radio frequency applications, particularly for use in an amplifier stage in a radio base station or in a ground transmitter for television or radio, to a power amplifier comprising said power transistor module and methods in the fabrication of the power transistor module and the power amplifier.
Power transistor modules for amplification at high frequencies have to comply with a number of detailed requirements as regards power amplification, ruggedness, break-down voltages, noise, distortion, capacitances, input and output impedances, etc. at a specified feed voltage and operation frequency. The demands on output power vary from a few watts to several hundred watts, in the latter case several components connected in parallel in a module being utilized. Power transistors operate at high signal levels and thus high current densities. The operation frequency is within the radio and microwave frequencies.
The power transistor modules are the critical components in power amplifiers in radio transmitters. The performance of the modules is the limiting factor as regards output power, efficiency and reliability.
In FIG. 1 is shown, schematically in cross-section, a power transistor module 1 mounted at a heat sink 3 and electrically connected to conductor pattern (not shown) on a circuit board or a printed circuit board 5 according to known practice, see e.g. U.S. Pat. No. 5,901,042 and references therein.
Module 1 comprises a rectangular electrically conducting and heat conducting bottom plate or flange 7, preferably of a metallic material, onto the upper surface of which are mounted a transistor chip 9 and typically a first 11 and a second 13 capacitor chip. Further, module 1 comprises an insulating, preferably ceramic, frame arrangement or isolator 15 mounted on the upper surface of flange 7 so that the arrangement surrounds said chips, and a cover 17 attached to frame 15.
The transistor chip 9 is typically a chip of the LDMOS (Lateral Double Diffused Metal Oxide) type and comprises a row of blocks connected in parallel, which each comprises a large number of transistor cells connected in parallel. The higher output power that is desired, the more transistor cells are needed. In FIG. 2 is shown a schematic layout of the transistor chip 9 comprising seven blocks. In each block all gates are connected in parallel to a gate connection or pad and all drains are connected to a drain connection or pad. In the uppermost block the gate pad is indicated by 19 and the drain pad is indicated by 21. The sources have connections at the back side/bottom side of the chip, which are connected to ground, e.g. via flange 7.
High power transistors have at high frequencies very low input and output impedances. To match these to surrounding circuits on the circuit board 5 impedance matching networks are needed close to the active chip with the transistors. Usually, these matching circuits are implemented by using bonding wires 23, 25, 27, 29 and capacitor chips 11, 13 inside the power transistor module 1. The bonding wires are further connected to straight butt connection contacts 31, 33, preferably of alloy 42, which are projecting from module 1 for connection to said surrounding circuits. In FIG. 2 bonding wires 25, 27 are indicated at the uppermost block of the chip. Each other block is certainly connected via its own bonding wires (not shown). Difficulties to attain tight tolerances during manufacture of the modules give rise to undesired variations in different critical electrical parameters.
The module 1 may be fastened to heat sink 3 by screws or bolts, whereafter the butt connection contacts 31, 33 are manually soldered to the surrounding circuits of circuit board 5 through soldered seams 35, 37. Flange 7 and heat sink 3 typically comprise apertures or recesses for receiving screws or bolts (not shown) for the mounting of module 1.
The matching network gives an extra contribution to the impedances before an outer impedance matching is performed on the circuit board. The placement of the transistor chip in the module is very critical, since it affects the length and form of the bonding wires and thereby the impedance matching. Length and form of the bonding wires may be difficult to define.
The low output impedance of the transistor chip is caused by the capacitance of the transistor chip together with a relatively low feeding voltage. The possibilities to achieve a good matching to the surrounding circuits are limited by these facts and by inductance in the conductors (particularly the bonding wires) between the power transistor module and the circuit board.
The impedance matching, which is required, therefore has to be performed both inside the power transistor module and outside on the circuit board. Variations in the properties of the transistor module, in the dimensions of the circuit boards and in the placement of the components will imply that a power amplifier comprising said components ought to be functionally trimmed to possess the best performance. Tolerance equalization through trimming, however, is not desirable by amplifier manufacturers. Manufacture without trimming will, however,-rarely provide for optimal performance but results is discarding of transistor modules.
Further, high power transistor modules of the above kind cannot be surface soldered (i.e. machine soldered) together with other surface soldered circuits on the circuit board and thereafter be mounted, i.e. fastened by screws, to a heat sink, since the modules are mechanically stressed during the screwing. The force arisen hereby is directly transferred to the soldered seams through the butt connector contacts, such that the soldered seams will be damaged, e.g. they will crack. This involves that the transistors have to be soldered manually as indicated, which is time-consuming, costly and results in large quality variations.
Due to the straight butt connector contacts, which are very rigid, the movements, which arise due to the temperature variations, will direct affect the soldered seams between the module and the circuit board. During these stresses the solder will plasticize and relax, which results in congruence displacements. When this is repeated sufficiently many times the soldered seams start to crack, which constitutes the main source of failure during operation. The lifetime of an amplifier will in this case be 5-10 years instead of the desired 30 years. Existing capsules for transistors have preferably a metallic flange and a ceramic isolator. Both the flange and the isolator must have a certain thickness due to purely mechanical reasons and the transistor module will therefore be thicker than the circuit board (which normally is 0.8 mm). A recess must therefore be milled in the heat sink in order to be able to lower the flange of the transistor module, see FIG. 1. This involves increased manufacturing expenditures and a risk for irregularities in the bottom of the recesses. Such an irregular mounting surface results in a risk that the capsules are bent during the fastening by screws, so that the chips crack. Further, the cooling is deteriorated.
It is an object of the present invention to provide a power transistor module intended for radio frequency applications, particularly for use in an amplifier stage in a radio base station, which is lacking at least some of the problems which are associated with prior art.
It is in this respect a particular object of the present invention to provide a power transistor module, which reduces the problems of high inductance and high resistance and of large parameter variations, which are associated with wire bonding.
It is a further object of the invention to provide a power transistor module, which offers possibilities to an enhanced impedance matching.
Further objects of the present invention will be apparent from the following description.
According to a first aspect of the present invention a power transistor module is thus provided, which comprises
a support plate;
a power transistor chip arranged on the support plate, the power transistor chip comprising an array of blocks connected in parallel, each of which comprising a plurality of transistors connected in parallel;
outer electrical connections for external connection of the transistor chip, the outer electrical connections projecting from the support plate; and
inner electrical connections connected between the transistor chip and the outer connections.
At least one block of transistors connected in parallel includes a plurality of connection pads for at least either ones of the gates or the drains of the transistors in the at least one block and at least one of the inner electrical connections comprises a first conductor pattern arranged on a flexible foil, wherein the first conductor pattern is connected between the plurality of connection pads and one of the outer connections.
Preferably, the first conductor pattern comprises an impedance matching network, particularly in the immediate vicinity of the power transistor chip. This network may comprise a plurality of capacitances and/or inductances. Further, at least some of the capacitances and/or inductances may comprise a trimmable, particularly laser trimmable, shunt for trimming the network for the purpose of reducing parameter variations that occur during the fabrication.
The flexible foil is preferably manufactured of an elastic polymer, e.g. a polyimide, and each conductor pattern is formed on at least one surface of the foil, preferably by printing or etching.
According to a further aspect of the present invention a power amplifier is provided comprising a power transistor module of the above-mentioned kind.
It is a further object of the invention to provide a method in the fabrication of the power transistor module.
According to yet an aspect of the present invention a method in the fabrication of a power transistor module is thus provided, comprising the following steps:
(1) providing a support plate;
(2) arranging a power transistor chip on the support plate, the power transistor chip comprising an array of blocks connected in parallel, each of which comprising a plurality of transistors connected in parallel and wherein at least one block of transistors connected in parallel is provided with a plurality of connection pads for at least either ones of the gates or the drains of the transistors in the at least one block;
(3) mounting outer electrical connections for external connection, the outer electrical connections are arranged in a manner so as to project from the support plate; and
(4) mounting inner electrical connections, the inner electrical connections being arranged to connect the transistor chip with the outer connections, wherein at least one of the inner electrical connections is provided as a first conductor pattern arranged on a flexible foil, the first conductor pattern being connected between the plurality of connection pads and one of the outer connections.
It is a further object of the invention to provide a power amplifier and a method in the fabrication of such a power amplifier.
According to further aspects of the present invention a power amplifier and a method in the fabrication of a power amplifier, respectively, are thus provided, which include the power transistor module, and the provision thereof, respectively.
An advantage of the present invention is that it provides for power transistor modules and power amplifiers with improved performances.
By using a flexible foil with a metallic pattern in accordance with the present invention one is not limited to solely one connection point per transistor block but it is possible e.g. to realize the connection at several points. This will give enhanced transistor performance, since the width of the access connectors may be reduced, which implies lower parasitic capacitances. The power distribution will become more uniform, since the outermost cells are active.
Yet an advantage of the present invention is that a faster, cheaper and more reliable fabrication method is achieved, in which parameter variations are minimized.
Further features and advantages of the invention will be apparent from the following detailed description of embodiments of the invention.